1. Field of the Invention
This invention relates generally to a transfer case for an all-wheel drive (AWD)/four-wheel drive (4WD) vehicle and, more particularly, to a transfer case for an AWD/4WD vehicle that employs a rear output shaft bushing that is cast with the transfer case housing, where the bushing incorporates a spiral groove for allowing lubricant flow therethrough.
2. Discussion of the Related Art
Various sport utility vehicles (SUV), off-road vehicles, four-wheel drive vehicles, etc. are equipped with drive modes that allow the vehicle to be driven in one or more of two-wheel drive high, four-wheel drive high, four-wheel drive low and AWD. Typically, these types of vehicles employ transfer cases that distribute the drive power received from an output shaft of the vehicle's transmission. Particularly, the output shaft of the transmission is coupled to an input shaft of the transfer case that distributes drive power to a front output shaft that is coupled to a front drive shaft that drives the vehicle's front wheels and a rear output shaft that is coupled to a rear drive shaft that drives the vehicle's rear wheels. Known transfer cases have employed various types of couplings, such as viscous couplings, electromagnetic clutches, positionable spur gears, etc., that allow the drive power from the transmission to be distributed to the front and rear drive shafts to provide the various drive modes.
The rear output shaft of the transfer case is coupled to the rear drive shaft by a slip yoke. The rear drive shaft is coupled to a rear axle of the vehicle, which is mounted to a vehicle suspension system. As the vehicle travels, the rear axle moves up and down in response to road conditions. As the rear axle moves up and down, the slip yoke slides on the rear output shaft so that the suspension load is not significantly transferred thereto. A bushing is typically employed in the opening of the transfer case housing through which the rear output shaft extends. The slip yoke is supported and rotates within the bushing, and is able to reciprocate therein along the axis of the output shaft. This allows the rear drive shaft to slide relative to the rear output shaft in response to rough driving conditions.
The housing of the transfer case is typically cast in two or more pieces and then bolted together. For example, a portion of the transfer case housing is sometimes cast as a separate cover housing and extension housing. An opening through which the rear output shaft extends is then machined into the appropriate housing piece to accept the bushing. The bushing is then pressed into the housing piece in a friction engagement before the housing pieces are bolted together. An inner surface of the bushing is then machined so that it has an internal diameter suitable for the outer diameter of the slip yoke.
It is desirable to limit the number of housing pieces to reduce costs and assembly time. However, if the housing piece to which the bushing is mounted is too large, then it becomes too difficult to machine the opening in the transfer case that accepts the bushing. Further, the bushing has a tendency to spin out of the opening in response to the load applied thereto from the slip yoke during operation. Also, the load from the slip yoke significantly increases the temperature of the bushing, that may lead to part failure of the bushing and/or rear output shaft.